The mechanism of catalysis of electron transfer from reduced pyridine nucleotide to disulfide substrates is being studied using yeast glutathione reductase and pig heart dihydrolipoyl dehydrogenase. Steady-state and stopped-flow kinetic techniques, together with static spectroscopic (absorption and fluorescence) studies, are being used to probe the sequence of individual steps involved in the overall catalytic reaction. In addition, a comparison is being made of the kinetic and catalytic properties of free dihydrolipoyl dehydrogenase with the dihydrolipoyl dehydrogenase component within the multienzyme pyruvate dehydrogenase complex in order to deduce the qualitative and quantitative advantages which may accrue upon elaboration of such multienzyme complexes.